FIG. 17(a) is a cross-sectional view showing a prior art ridge waveguide type semiconductor laser device. In FIG. 17(a), reference numeral 400 designates an n type GaAs series semiconductor laser device. An n type AlGaAs first cladding layer 2 is disposed on the n type GaAs substrate 1. A p type AlGaAs active layer 3 is disposed on the n type AlGaAs first cladding layer 2. A p type AlGaAs second cladding layer 4 having a ridge in a reverse mesa shape is disposed on the p type AlGaAs active layer 3. A p type GaAs first cap layer 5 is disposed on the ridge of the second cladding layer 4. An n type GaAs current blocking layer 8 and a p type GaAs second cap layer 50 are disposed at both sides of the ridge. A p type GaAs contact layer 9 is disposed on the p type GaAs second cap layer 50. A p side electrode 10 and an n side electrode 11 are disposed on the p type GaAs contact layer 9 and the rear surface of the n type GaAs substrate 1, respectively. FIG. 17(b) shows an n type GaAs semiconductor laser device having the same structure as the device of FIG. 17(a), in which the p type GaAs contact layer 9 is thicker than that of FIG. 17(a).
A description is given of the production method. FIGS. 18(a)-18(e) illustrate process steps in a method for producing the semiconductor laser shown in FIG. 17(a). Initially, as illustrated in FIG. 18(a), there are successively grown on the n type GaAs substrate 1 the n type AlGaAs first cladding layer 2, the p type AlGaAs active layer 3, the p type AlGaAs second cladding layer 4, and the p type GaAs first cap layer 5 (first epitaxial growth step). Preferably, these layers are grown by metal organic chemical vapor deposition (MOCVD).
Then, as illustrated in FIG. 18(b), an SiN film 6 and a photoresist 7 are successively deposited on the first cap layer 5, and the SiN film 6 is patterned in a stripe shape by conventional photolithographic and selective etching steps.
Then, as illustrated in FIG. 18(c), after removing the photoresist 7, the p type AlGaAs second cladding layer 4 and the p type GaAs first cap layer 5 are selectively removed by wet etching using the SiN film 6 as a mask, leaving a ridge having a reverse mesa shape. The wet etching should be carried out so that the p type AlGaAs second cladding layer 4 remains on the p type AlGaAs active layer 3 at opposite sides of the ridge.
Subsequently, as illustrated in FIG. 18(d), the n type GaAs current blocking layer 8 and the p type GaAs second cap layer 50 are successively grown on the p type AlGaAs second cladding layer 4 to bury the ridge, preferably by MOCVD (second epitaxial growth step).
After removing the SiN film 6, the p type GaAs contact layer 9 is grown on the p type GaAs first cap layer 5 and the p type GaAs second cap layer 50, preferably by MOCVD (third epitaxial growth step). Thereafter, the p side electrode 10 and the n side electrode 11 are formed on the p type GaAs contact layer 9 and the rear surface of the n type GaAs substrate 1, respectively, completing the semiconductor laser device 400 shown in FIG. 18(e). If the p type GaAs contact layer 9 is grown thick in the third epitaxial growth step, the laser structure shown in FIG. 17(b) is obtained.
In operation, when a forward bias voltage is applied across the n type GaAs substrate 1 and the p type GaAs contact layer 9, current flows into the p type AlGaAs active layer 3 through the ridge having a reverse mesa shape and carriers are confined in the p type AlGaAs active layer, resulting in carrier recombinations that produce laser light. At this time, since light absorption and current concentration are caused by the n type GaAs current blocking layer 8, a difference in refractive index along in the horizontal direction of the active layer 3, restricting the extent of the generated light in the transverse direction. Thus guided light resonates in a Fabry-Perot resonator which is constituted by cleavage facets opposite to each other in the longitudinal direction of the stripe-shaped ridge, and laser oscillation occurs.
The conventional ridge waveguide type semiconductor laser device has the following drawbacks.
FIG. 19 is a sketch of an electron microscope photograph illustrating a cross-section of the wafer after the second epitaxial growth step shown in FIG. 18(d). During the selective etching for forming the ridge structure having a reverse mesa shape, portions of the cap layer 5 beneath the ends of the SiN film 6 serving as an etching mask are unfavorably etched away, resulting in over-hanging portions 6a. This is caused by poor adhesion between the etching mask, comprising SiN or the like, and the epitaxially grown layer so that the etchant permeates into the interface between them. If such over-hanging portions 6a are present in the second epitaxial growth step, i.e., the step of successively growing the n type GaAs current blocking layer 8 and the p type GaAs second cap layer 50 on the p type AlGaAs second cladding layer 4, reactive gases do not reach the over-hanging portions 6a, resulting in an uneven growth that produces hollows 21.
When the SiN film 6 is removed and the p type GaAs contact layer 9 is grown on the first and second cap layers 5 and 50 by the third epitaxial growth step, concave portions are formed on the surface of the contact layer 9. In addition, since the crystal growth is unevenly carried out beneath the over-hanging portions 6a, crystallinity of the p type GaAs contact layer 9 disposed thereon is poor, adversely affecting performance of the semiconductor device. In addition, if concave portions are formed on the p type GaAs contact layer 9, a metal film, serving as a p side electrode 10, is not evenly deposited on the contact layer, resulting in an unreliable device. Especially when the p type GaAs contact layer 9 is thin as shown in FIG. 17(a), the p type GaAs contact layer 9 is unfavorably broken due to the concave portions between the p type GaAs first cap layer 5 on the ridge and the p type GaAs second cap layer 50, and the p side electrode 10 on the contact layer 9 is also broken, resulting in further reduction in reliability of the device.
If crystallinity of the epitaxially grown layer, which is regrown at opposite sides of the ridge, deteriorates, the thickness of the epitaxially grown layer is not even and the ridge unfavorably protrudes. In this state, when the p type GaAs contact layer 9 is formed, a convex portion is formed on the surface of the p type GaAs contact layer 9 and a stress is applied to the ridge in the subsequent steps, such as a step of polishing the rear surface of the substrata, a step of mounting the semiconductor chip on a package with solder while connecting the ridge side to a heat sink (junction down), and the like, whereby the ridge is damaged. In addition, the convex portion of, a, the contact layer does not adhere closely to the heat sink, causing an inclination of a laser beam produced by the chip.
Meanwhile, Japanese Published Patent Applications Nos. 63-269593 and 1-287980 and Mitsubishi Denki Giho Vol.62, No. 11 (1988), pp.958 to 961 propose ridge waveguide type semiconductor laser devices in which a p type AlGaAs buffer layer or a p type GaAs buffer layer is grown on a p type AlGaAs cladding layer exposed at both sides of a ridge and then an n type GaAs current blocking layer is grown. In these laser devices, since the crystal growth of the p type AlGaAs or GaAs buffer layer smoothly progresses on the p type AlGaAs cladding layer, the crystal layer is grown to some degree beneath the above-described over-hanging portions. However, it is impossible to completely fill up the hollows under the over-hanging portions, so that the above-described problems have not been completely solved yet.
FIG. 21 is a graph showing oxygen concentrations determined by SIMS analysis in the p type AlGaAs second cladding layer 4 and the n type GaAs current blocking layer 8 produced in the process steps shown in FIGS. 18(a)-18(e). Since the surface of the p type AlGaAs second cladding layer 4 is exposed to the atmosphere during the etching for forming the ridge, much oxygen exists on the surface of the cladding layer 4. As the result, an interface level is produced between the cladding layer 4 and the current blocking layer 8 in a completed device, and leakage current generated in this region, deteriorating characteristics of the device.
When the ridge is formed by wet etching, fine grooves are formed on the exposed surface of the p type AlGaAs second cladding layer 4. The depth of each groove is equal to a thickness of several atom layers, i.e., 10 to 20 angstroms. When the n type GaAs current blocking layer 8 is epitaxially grown on the p type AlGaAs second cladding layer 4, since the n type GaAs does not favorably match with the p type AlGaAs, the grooves also appear on the surface of the n type GaAs current blocking layer 8, whereby crystallinity of the current blocking layer 8 deteriorates, resulting in deterioration in the device characteristics.
As a solution of the above-described problem, there is a method for producing a ridge waveguide type semiconductor laser device disclosed in Japanese Published Patent Application No. 64-84780. In this method, after a ridge is formed by wet etching, the surface of a first semiconductor layer exposed at both sides of the ridge is etched by vapor phase etching in a reaction tube, and a second semiconductor layer is grown on the first semiconductor layer in the same reaction tube. In this case, although the concentration of oxygen on the crystal surface is reduced and the grooves generated on the crystal surface due to the wet etching are decreased, since the vapor phase etching and the crystal growth are carried out in the same reaction tube, reaction products of the vapor phase etching adversely affect the crystal growth, resulting in poor crystallinity of the grown layer.
FIGS. 20(a)-20(c) are cross-sectional views of process steps in a method for dividing a semiconductor substrate, on which a plurality of p type InP ridge waveguide semiconductor laser elements are disposed, into a plurality of semiconductor laser devices. In the figures, semiconductor laser elements 19 and 20 are adjacent to each other on a p type InP substrate 12. In each semiconductor element, a ridge 22 is formed in the substrate 12 and an undoped InGaAsP active layer 14 is formed in the ridge 22. An InP epitaxial layer 13 is disposed on the p type InP substrate 12 at opposite sides of the ridge 22. An SiO.sub.2 film 15 is disposed on the entire surface except for the center of the top surface of the ridge 22. An n side electrode 16 is disposed on the ridge 22 and the SiO.sub.2 film 15. Reference numeral 18 designates an interface between the p type InP substrate 12 and the InP epitaxial layer 13 and numeral 17a (17b) designates a photoresist.
First of all, as illustrated in FIG. 20(a), a photoresist 17a is deposited on the entire surface of the p type InP substrate 12, on which the semiconductor elements 19 and 20 are present, and patterned to form a stripe groove approximately 5 microns wide between the elements 19 and 20. Then, using the photoresist 17a as a mask, the SiO.sub.2 film 15 is etched away to expose the surface of the epitaxial layer 13.
Then, as illustrated in FIG. 20(b), using the SiO.sub.2 film 15 as a mask, the InP substrate 12 and the epitaxial layer 13 are wet etched using an etchant including hydrochloric acid to form a groove 21 deeper than the interface 18. In this case, hanging portions 15a about 2 microns formed on both ends of the groove 23 because of the side etching of the layers 12 and 13. If the wafer is divided without removing the over-hanging portions 15a, SiO.sub.2 powder is produced and adheres to the pn-junction exposed on the cleavage plane of the separated element, causing a leakage current. Or, the over-hanging portions of the SiO.sub.2 film 15 come off, adversely affecting the characteristics and reliability of the laser device.
In order to avoid this problem, the over-hanging portions 15a may be removed before the division of the wafer. More specifically, after removing the photoresist 17a, a photoresist 17b is deposited on the entire surface of the wafer and patterned to form an aperture of approximately 30 microns width above the groove 21. Using the photoresist 17b as a mask, the over-hanging portions 15a of the SiO.sub.2 film 15 are etched away by an etchant including HF as shown in FIG. 20(c). Finally, the p type InP substrate 12 is cut along the groove 21 to separate the semiconductor laser elements 19 and 20 from each other.
In this method, however, the step of removing the over-hanging portions 15a is very complicated. In addition, since it is difficult to precisely remove only the over-hanging portions 15a, the end surface of the InP epitaxial layer 13 is unfavorably exposed, resulting in deterioration in the device characteristics.